In fluoroelastomeric compositions the use of coadjuvants is necessary because of the poor characteristics of extrudability of the compounds, of the phenomena of adhesion to the molds, and of fouling thereof.
In the fluoroelastomeric compositions of the prior art, as plasticizers or lubricant products use was made of the following types: vegetable waxes, low-molecular-weight polyethylene, miscellaneous stearates, polyesters, octadecylamine and so forth.
The processing coadjuvants of the prior art cause, in some cases, problems of interference with the peroxidic vulcanization system, and problems of poor surface appearance of the molded article. Furthermore, in the specific case of fluoroelastomers, for the purpose of endowing them with the well-known properties of heat resistance and resistance to chemical agents and solvents, a "thermal stabilization" (post-cure) at a temperature of from 200.degree. to 250.degree. C. for a time of from 10 to 30 hours is essential. During this step, the common plasticizers (polyglycols, stearates, organic phosphates, oleates, phthalates, etc.) volatilize and/or decompose.
This causes a considerable deterioration of the mechanical properties of the finished articles; in particular:
excessive increase in hardness; PA1 decrease of compression set; PA1 decrease of the elastic characteristics (low elongation and high modulus value) PA1 poor heat resistance, which prevents the articles from being used under severe application conditions (T&gt;180.degree. C.). PA1 n=integer of from 1 to 30; PA1 m=integer of from 0 to 10; PA1 p and q are integers within the range of from 0 to 20, PA1 high processability, in particular as relates to the processing by extruder; PA1 improved low-temperature properties of the vulcanizate (low T.sub.g); PA1 enhanced compatibility of the fluoroelastomers with non-fluorinated elastomers, with an improved vulcanization rate of such blends; PA1 improved elastomeric properties of the cured article; PA1 better thermal and chemical stability of the cured article.
Summing up, the benefits obtained during the processing step correspond to unacceptable drawbacks in the finished articles.
As a consequence, the use of plasticizers or lubricating agents is limited to low amounts (1-2 p.h.r.), which do not sensibly affect the end properties of the cured articles, but which are, on the other hand, insufficient for fulfilling the above requirements.
It was observed, in particular, that the use of perfluorosiliconic oils allows the hardness to be reduced by 4-5 points; it shows, however, difficulties for the blending and incorporation of said oils into the fluoroelastomer, due to the mutual incompatibility of the two polymeric systems. Furthermore, a lowering in curing rate and a deterioration of compression set value and of thermal resistance are observed.
The use is known as well of low-molecular-weight fluorinated copolymers of vinylidene fluoride, eventually in combination with the above indicated plasticizers, to endow the compounds with lower Mooney viscosity values, and better rheological characteristics during the processing and fabricating steps (see Du Pont's Viton.RTM.).
However, such a type of plasticizers can only be used with curing system having ionic mechanisms and which use diaminic or bisphenolic derivatives, in the presence of an accelerating agent (U.S. Pat. No. 3,712,877 by 3M).
By this curing system, the plasticizer polymer having low-molecular-weight is bonded to the fluoroelastomer during the vulcanization process.
On the contrary, no chemical bonds with the same plasticizer (type VITON.RTM. LM) are formed (*), if the fluoroelastomers are formulated for curing with peroxidic systems. From this, there results a worsening in the characteristics of stability to acids, to steam and to heat, in polar solvents and to SF-type formulated oils, such as BP olex, characteristics which are typically required in fluoroelastomers vulcanized with peroxides. (*) see VITON data sheet VT 240 LM (R1).